Not Applicable
Not Applicable
The invention relates to a process for the manufacture of compressed articles, particularly cemented-carbide reversible cutting blade inserts, by compressing metallic powder and subsequently sintering the compact.
It is known to manufacture blanks from cemented carbide, ceramic material, sintered metal or the like by means of presses. The powdered or granular material requires to be provided in such a manner that the compact, when under an applied compacting pressure, assumes a homogeneous structure and allows itself to be sintered. A common forming operation is the so-called direct pressing process in appropriately designed die-sets or die-plates with which a top ram and a bottom ram are associated. In accordance with the respective compacting pressure, different densities will result for the compact. Lower-density compacts, however, will shrink more than higher-density compacts do during sintering. An attempt is made to minimize variations in density by means of differently adjustable compressing strokes for the top and bottom rams. On the other hand, in practice, varying densities may arise because of varying compressive forces which, in turn, are provoked, for example, by charging variations which may amount to some per cents with the compact heights being the same. A difficulty in manufacturing compacts, e.g. for cemented-carbide reversible cutting blade inserts, is that a predetermined overall height is maintained between the cutting blade insert seating and at least one cutting edge is of a predetermined distance from the cutting blade insert seating.
It has become known from DE 42 09 767 to achieve a density as uniform as possible within a batch, for example, by measuring the compressive force and subsequently making a correction via the charging volume for the compacts that succeed.
Further, it has become known from DE 197 17 217 to determine and store a desired force-stroke diagram (a desired curve), which is dependent on the geometry of the compact and the base material, for a compacting ram during compression. Using a separately operated portion of the compacting ram or a separate ram, the pressure acting on the material to be compressed is increased or decreased during compression as soon as a deviation from the desired curve is found to exist with a view to attaining the same density for each compact at the end of the compression phase. A procedure of this type, however, can only be applied to compacts in which the surface of the compact is uncritical in the medium range. For example, this applies to the seating surface in which it is sufficient, for example, for a circumferential edge to be at a precise distance from the cutting edge whereas the medium range may be more or less recessed.
It is the object of this invention to provide a process for the manufacture of cemented-carbide cutting blade inserts by compressing and sintering the compact, particularly cemented-carbide reversible carbide cutting blade inserts, which is simpler than the known, previously described process and, notwithstanding this, leads to excellent results.
Like in the conventional process, the invention also provides for a predetermined volume of metallic powder to be charged into the die-plate bore with the bottom ram taking a charging position here. If required, the bottom ram is initially moved to a somewhat lower position so that a slight excess volume is charged, after which the bottom ram will take its final charging position and the remaining volume expelled from the die-plate bore will be stripped off by means of the charging shoe. Subsequently, the top ram and the bottom ram are moved to predetermined first and second positions, respectively, wherein a certain compressive force may be applied already. If it concerns a compact which is supposed to be used for a cutting blade insert with a clearance angle the location of the top ram, at the second position to which it is moved, corresponds to the upper edge of the compact, for example. Another shift of the bottom and top rams is effected subsequently, in which instance, however, only the bottom ram is displaced in case of a compact with a clearance angle. Compressive forces are continuously measured during this shifting operation where the infeed movement of the bottom and top rams is terminated once the compressive force has reached a predetermined value. Even if each of the two rams is displaced it may be sufficient to measure the compressive force on the bottom ram alone and to terminate the shifting motion once the compressive force has reached the predetermined value.
The value predetermined for the compressive force is determined by preceding trials. Initially, a determination is made as to which compaction the metallic powder is to undergo in order to be subjected to the sintering process afterwards. Then, an investigation is made on what the magnitude of the charging volume should be in order that a determined height of the compact be reproducibly achieved if a predetermined compressive force is applied. Therefore, if a turn-off is made once a predetermined compressive force is reached in the inventive process an assumption can be made that the predetermined height of the compact has been reached. In this way, a predetermined density of the compact is attained even in case of certain charging level variations exist. Since charging level variations cannot completely be precluded the preferred procedure is such that if tolerances exist there is a certain excess volume, when in doubt, if a turn-off is made at a predetermined compressive force value. In case of an excess volume, the compact is reduced in height, preferably by grinding, in order to bring it to the predetermined height or thickness.
According to an aspect of the invention, the shift of the bottom ram and/or the top ram from the first and second positions, respectively, is performed along a predetermined curve desired for the compressive force, which desired curve reflects the dependence of the compressive force on time. Thus, it becomes possible to approach the final compressive force values desired in a regulating way.
Although the compressive force is also measured in the known process, but while a predetermined position is moved to in order to vary the charging volume in case of compressive force deviations afterwards, the invention provides for a correction to be made directly on the compact.
The invention will now be explained in more detail with reference to drawings.